1. Field of the Invention
The present invention relates to a method for dry-etching of silicon substrate, used in production of semiconductor device.
2. Description of the Prior Art
In recent years, LSI devices have been required to have fineness in order to achieve a high degree of integration. In this connection, fineness is required as well in formation of isolation regions. Hence, it has been considered to form isolation by making narrow and deep grooves in a substrate and then filling up the grooves with an insulating material. In this approach, voids must not be formed in the grooves at the time of filling up the grooves with an insulating material, in order to produce a reliable device.
A method for preventing formation of voids when an insulating material is filled up in the grooves, is described in Japanese Patent Application Laid-Open (Kokai) No. 1-107554. The method is explained with reference to FIG. 7. First, as shown in FIG. 7(a), a silicon oxide film 702 is formed on a silicon substrate 701 by patterning, only at the substrate areas where grooves are to be formed. Then, as shown in FIG. 7(b), isotropic plasma etching is conducted to the silicon substrate 701 using the silicon oxide film 702 as a mask, whereby dents having an arc-shaped section are formed. As shown in FIG. 7(c), anisotropic etching is conducted to each dent using the silicon oxide film 702 as a mask, to form a groove 703 of desired depth. Next, as shown in FIG. 7(d), the silicon oxide film 702 (mask) is removed, whereby the groove 703 comes to have a sectional shape having a taper 704 at the opening. Into the groove 703 is filled up a silicon oxide film 705 and then a flattening technique by etch back is applied, whereby is completed, as shown in FIG. 7(e), isolation by a groove 703 filled up with a silicon oxide film 705. Thus, formation of taper 704 at groove opening makes smooth the filling of silicon oxide film 705 and can prevent appearance of voids.
A method for preventing formation of voids at the time of filling an insulating material into a groove, is also described in Japanese Patent Application Laid-Open (Kokai) No. 58-9333. In the method is formed a groove of tapered sectional shape having an inclination of about 30-65.degree. at the top and an inclination of about 70-90.degree. at the bottom.
In each of these two methods for groove formation, isotropic etching is conducted in the first etching for silicon substrate Japanese Patent Application Laid-Open (Kokai) No. 1-107554! and in the etching for obtaining inclination at groove top Japanese Patent Application Laid-Open (Kokai) No. 58-9333!. Therefore, the amount of etching in horizontal direction must be taken into consideration at the stage of device designing, which makes it difficult to obtain a device of high integration and fineness.
In Japanese Patent Application Laid-Open (Kokai) No. 63-124420 is described a method of forming each groove without resorting to isotropic etching and then filling an insulating film in the groove without forming voids in the groove. In this method, there is used, as an etching gas, a gas capable of forming a deposit upon plasma polymerization, or a gas capable of forming a deposit by reacting with an etching product; the gas pressure is set at less than 0.01 Torr; the sample temperature is kept lower than 70.degree. C.; thereby, the groove formed is allowed to have a tapered sectional shape so that an insulating film can be filled in the groove in an improved state.
Other method for forming each groove of tapered sectional shape is described in Japanese Patent Application Laid-Open (Kokai) No. 2-84721. In this method, etching is conducted using, as an etching gas, a mixed gas containing boron chloride and nitrogen.
Each of the above two methods has a problem. In the method described in Japanese Patent Application Laid-Open (Kokai) No. 63-124420, intended etching is possible when a silicon oxide film is used as a mask material, but is difficult when a photoresist is used. This is because the reaction product formed during etching differs depending upon the mask material used and has a largely different protective action for the side wall of groove. SiOx (when the mask is silicon oxide film) or SiCx (when the mask is a photoresist) is contained in the reaction product and deposits on the side wall of groove. SiCx, as compared with SiOx, has a small protective action for the side wall of groove; accordingly, it is difficult to form a vertical groove of tapered sectional shape by using a photoresist. In the method described in Japanese Patent Application Laid-Open (Kokai) No. 2-84721, the side wall-protecting film formed comprises boron and nitrogen, is very hard and strong, and is very hard to remove after etching.